In an AMOLED display device, the luminescence brightness of each OLED is determined by driving current generated by a driving circuit, and the driving current generated by the driving circuit can be expressed by the following formula:IOLED=K·(Vgs−Vth)2,
wherein K is a parameter related with the process parameter and the feature size of a driving transistor, Vgs is the voltage difference between the gate and the source or between the gate and the drain of the driving transistor (determined by the type of the driving transistor), and Vth is the threshold voltage of the driving transistor.
When the AMOLED display device displays a frame of picture, the threshold voltage Vth of the driving transistor may drift, and the driving current IOLED is changed, so that the luminescence brightness of the OLED is changed, to influence the display uniformity of single pixels in a frame of picture (particularly for an LTPS type display device). Moreover, the drift amplitudes of the threshold voltages of the driving transistors in different pixel units may be different under the action of long-time high temperature and high voltage, which leads to difference of display brightness, and such brightness difference is related with the image of the previous frame of picture, so that a “ghost” phenomenon is finally produced.
On the other hand, in the AMOLED display device, power supply lines inherently have certain resistance, so that a power supply voltage input to the pixel unit close to a power supply is relatively high, whereas the power supply voltage input to the pixel unit far from the power supply is relatively low, namely a so-called “voltage drop” (IR Drop) phenomenon appears. This phenomenon also influences the display uniformity of the pixel unit in each area of the AMOLED display device.
Moreover, in the evaporation process of fabricating process for the OLED device, the thickness of film formed by evaporation has certain differences among different OLED devices, and the non-uniformity of the film thickness in the OLED devices also influences the display uniformity of the pixel units.